The present invention relates to a method for the diagnosis of an internal combustion engine exhaust gas system that includes a particle filter. In particular the invention provides a method for determining a flow anomaly in the exhaust gas system, based on a volume flow dependence of pressure values in the exhaust gas system.
The use of particle filters to remove particles from the exhaust gas of motor vehicle internal combustion engines is common. However, such filtering leads to clogging of the particle filter over time, so that flow-through resistance increases in an undesired manner. For this reason, it is common to carry out a regeneration of the filter from time to time, so that soot particles collected in the particle filter are burnt off.
The necessity of a particle filter regeneration is often determined by evaluating the value of differential pressure across the particle filter. German patent document DE 42 26 055 C1, for example, discloses a method in which the differential pressure over the particle filter is measured and compared to a differential pressure of a constant throttle location in the exhaust gas system. In this manner, the influence of a (usually present) lack of stationarity of the exhaust gas flow with correspondingly alternating differential pressure ratios is considered, and a conclusion can be drawn with regard to the loading degree of the particle filter, essentially independent from the exhaust gas volume flow.
German patent document DE 10 2004 026 589 A1 discloses a method for monitoring a particle filter by which both the loading state and breakage of the filter can also be determined. For this purpose, a filter loading value is determined based on determined pressure loss values across the particle filter and an exhaust gas volume flow through the particle filter; and its temporal development is viewed. If it is determined that the derivative of the filter loading value is less than zero, a filter breakage is diagnosed.
One object of the present invention is to provide a method which permits more comprehensive diagnostics of an internal combustion engine exhaust gas system comprising a particle filter.
This and other objects and advantages are achieved by the method according to the invention, in which first and second sets of a plurality of value vectors are determined, wherein the values of each value vector are associated with each other and represent i) an absolute pressure upstream of the particle filter, ii) an absolute pressure downstream of the particle filter, iii) a differential pressure (determined from the values of the absolute pressure upstream and downstream of the particle filter), iv) an exhaust gas volume flow through the particle filter, and v) a temperature of the exhaust gas volume flow, respectively. The determined values for the absolute pressure upstream and downstream of the particle filter, and the differential pressure at least of the first set of value vectors are transformed to temperature-standardized values so that temperature dependence is eliminated, or nearly eliminated. Based on the temperature-standardized values of the first set of value vectors, characteristics are generated for dependence of the absolute pressure upstream and downstream of the particle filter and the differential pressure, on the volume flow. These characteristics are stored as reference characteristics.
Values for the absolute pressure upstream and downstream of the particle filter and the differential pressure of the second set of value vectors, are compared to corresponding pressure values derived from the reference characteristics, and the result is evaluated with regard to a flow anomaly in the exhaust gas system. Leakage or clogging of the exhaust gas system (or a component installed therein) is thus understood as a flow anomaly.
A set of value vectors typically comprises from about a hundred up to a few thousand value vectors and is determined over a longer period of time (typically 5 min to 50 min). In practice, the determined values for the absolute pressure upstream and downstream of the particle filter are thus within a relatively broad value region for the exhaust gas volume flow, and it is not difficult to generate pressure or differential pressure volume flow characteristics therefrom. This is advantageous for the reliability of the diagnostics method, as the reliability of the diagnostics result is improved due to the plurality of considered values. The values for the absolute pressure upstream and downstream of the particle filter, the exhaust gas flow and the associated temperature are thereby preferably synchronized and determined at least approximately simultaneously, and stored as components of a respective value vector.
A further improvement of the reliability is obtained by eliminating the physically caused temperature dependence of the pressure values according to the invention. In this way, interpretable pressure values can be determined, even with typically changing flow and temperature ratios for motor vehicle uses. The temperature dependence of the pressure values is preferably eliminated by considering the parameters for exhaust gas density and dynamic viscosity of the exhaust gas. For these parameters, temperature dependence is preferably represented in the form of a characteristic. Together with the exhaust gas temperature sensed parallel to the pressure values, the formation of temperature-standardized pressure values is thus facilitated.
To determine the exhaust gas volume flow, the fuel and combustion air amounts implemented per unit of time are preferably consulted. These values are often available in an engine control device in any case. The values for exhaust gas volume flow are preferably also standardized to preset pressure and/or temperature reference conditions, so that dependencies of pressure and/or temperature are also eliminated.
Preferably, the standardized exhaust gas volume flow values are used to generate the characteristics or reference characteristics. The absolute pressure values downstream and upstream of the particle filter are preferably determined in a measurement-technological manner using suitably arranged absolute pressure sensors. The absolute pressure sensors can be arranged immediately on the inlet side or the outlet side of the particle filter body, or also a bit farther upstream or downstream in the exhaust gas system. The differential pressure can be obtained simply by subtraction of the absolute pressure values. The temperature of the exhaust gas flowing through the particle filter is conveniently sensed in a measurement-technological manner, using a temperature sensor arranged on the inlet side and/or outlet side of the particle filter and/or in the particle filter.
The second set of value vectors is preferably determined temporally following the first set. Values of the second set, which were determined with preset or presettable conditions with regard to preset or presettable exhaust gas and/or internal combustion engine operating parameters, or fall within preset or presettable value regions, are preferably consulted for comparison with pressure values derived from the reference characteristics. A comparison of for example differential pressure values of the second set of value vectors with pressure values derived from the reference characteristic for the differential pressure preferably entails comparing a plurality of value pairs for the differential pressure and the associated exhaust gas volume flow with the corresponding value pairs of the reference characteristic for the differential pressure.
If the differential pressure values of the second set of value vectors are outside a preset or presettable confidence interval relative to the reference characteristic values, a flow anomaly is possibly diagnosed. (For this comparison, temperature-standardized or de-standardized pressure values can be consulted; pressure values of the second set of value vectors with corresponding pressure values calculated back from reference characteristic values related to the present temperatures are preferred.) If no irregularities are determined during the evaluation of the comparison result, new characteristics or reference characteristics are generated from the values of the second set of value vectors, and stored. In this manner, updated characteristics which are adapted to the respective conditions are continuously available. This has the advantage that erroneous diagnostics due to long term drift effects can be avoided and normal gradually occurring changes are not erroneously diagnosed as a flow anomaly,
The determination of three different pressure values (that is, the absolute pressure on the inlet side and the outlet side of the particle filter, and the differential pressure over the particle filter) provided according to the invention is especially advantageous. It facilitates a differentiating evaluation at least with regard to leakage of an exhaust gas line section downstream of the particle filter, leakage of an exhaust gas line conducting section upstream of the particle filter, and a breakage of the particle filter in the arrangement of the invention, Especially, evaluation of comparison results obtained separately for the two absolute pressure values and for the differential pressure with regard to the stored reference characteristic values, facilitates a differentiation of the mentioned flow anomalies. This technique has proved to be particularly advantageous in practice, because it avoids an unnecessary change of a particle filter erroneously diagnosed as being broken. Such an erroneous diagnosis is especially possible, if a leakage occurs in an exhaust gas line section upstream of the particle filter, with a lowering of a back pressure on the inlet side of the particle filter resulting therefrom. With an exclusively differential pressure sensing, for example by a differential pressure sensor, the danger of an erroneously diagnosed filter breakage becomes large, with a corresponding high probability of incurring the cost of an unnecessary filter change.
In a further embodiment of the method according to the invention, clogging of the particle filter and/or an exhaust gas cleaning unit connected upstream or downstream of the particle filter is evaluated. For recognizing these flow anomalies, the determination of the three different pressure values provided according to the invention is again advantageous. In particular, determination of the absolute pressure downstream of the particle filter makes it possible to distinguish between a filter breakage and a clogged exhaust gas cleaning unit arranged downstream of the particle filter. If only differential pressure is sensed, an unacceptably high absolute pressure downstream of the particle filter but upstream of the exhaust gas cleaning unit connected downstream, could be diagnosed erroneously as filter breakage, despite the fact that clogging of the exhaust gas cleaning unit downstream of the particle filter is the more probable flow anomaly.
The process according to the invention facilitates a goal-oriented and successful initiation of measures for removing the flow anomaly. The recognition of clogging occurring downstream of the particle filter is especially advantageous when using a downstream SCR catalyst, because the danger of clogging by urea deposits due to a supply of aqueous urea solution is increased in that case. An exhaust gas cleaning unit arranged upstream of the particle filter is preferably formed as oxidation catalyst. A clogging can occur in this case by fuel coking, for example due to a fuel addition for an enforced particle filter regeneration.
In a further embodiment of the method according to the invention, for generating the characteristics for the dependence of the absolute pressure on the inlet side and the outlet side of the particle filter and of the differential pressure of the volume flow, temperature-standardized values of the first set of value vectors are fed to a Kalman filter algorithm. Starting with the knowledge that at least approximately a normal (Gaussian) distribution exists for the uncertainty of the sensed values, the application of the Kalman filter algorithm facilitates the determination of characteristics with an error as small as possible. In other words, due to the procedure according to the invention, the most probable characteristics are determined, whereby the evaluation of the comparison results is facilitated with a maximum reliability.
In a further embodiment of the method according to the invention, for generating the characteristics only those values for the absolute pressure upstream and downstream of the particle filter and the differential pressure of the first set of value vectors are used, which were obtained in a first presettable region for exhaust gas volume flow and/or a presettable region for temperature of the exhaust gas volume flow, thereby further enhancing the probable accuracy of the diagnostics result. A region between 50 m3/h and 500 m3/h is preferably preset for the volume flow for an internal combustion engine with a displacement of 10 l, and a region between 150° C. and 550° C. is preferably preset for the temperature of the exhaust gas volume flow. The value region for the exhaust gas absolute pressure can also be preset. A mutual dependence of the allowed value regions of exhaust gas temperature, pressure and volume flow can also be provided.
A further improvement of the diagnostics result can be achieved if only those values for absolute pressure upstream and downstream of the particle filter and the differential pressure of the second set of value vectors which were obtained in a second presettable region for the exhaust gas volume flow are taken into account for the purpose of comparison with pressure values derived from the reference characteristics, to determine a flow anomaly. The value region is preferably chosen smaller with regard to a preset or a first presettable value region. A value region with a lower limit of at least 600 m3/h to 800 m3/h is preferred. Further limitations can of course be provided for the use of values of the second set of value vectors for the purpose of comparison. In particular, a virtual steady state of the operation of the internal combustion engine can be requested, which is defined by an admissible fluctuation range for the speed and/or released torque.
According to a further embodiment of the method, characteristics obtained from values which were determined in a presettable temporal distance after a first startup of the particle filter and/or after a forced particle filter regeneration are used as reference characteristics. In this manner, defined comparison conditions are provided, or inlet or start-up effects are faded out.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.